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Bioresource Technology 96 (2005) 1093–1101
Delayed nutrient application affects mineralisation rate duringcomposting of plant residues
Dorte Bodin Dresbøll a,b,*, Kristian Thorup-Kristensen a
a Department of Horticulture, Danish Institute of Agricultural Sciences, Kirstinebjergvej 10, DK-5792 Aarslev, Denmarkb Plant Nutrition and Soil Fertility Laboratory, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University,
Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark
Received 27 February 2004; received in revised form 12 October 2004; accepted 22 October 2004
Available online 8 December 2004
Abstract
The hypothesis that delayed addition of nutrient rich material to compost would influence the mineralisation pattern was inves-
tigated by studying N turnover in compost based on wheat straw and clover-grass hay. After 7 12weeks of composting almost twice as
much N was mineralised when the addition of some of the N-rich clover-grass hay was postponed, suggesting that this influenced the
microbial succession. The delayed addition resulted in a second temperature peak and a decline in the pH. Despite the altered con-
ditions no significant effect was observed on the weight loss or loss of C and N. In conclusion, compost processes can in a simple way
be affected by delayed substrate application leading to a higher nutrient availability without altering other parameters significantly.
� 2004 Elsevier Ltd. All rights reserved.
Keywords: Composting; Nitrogen mineralisation; Growing medium; Plant residues; Decomposition
1. Introduction
Composting experiments have been performed inten-sively during the last decades. Primarily the studies have
focused on rural and urban wastes, often with the aim of
reducing volume and avoiding nutrient losses (Witter
and Lopez-Real, 1988; Martins and Dewes, 1992; San-
chez-Monedero et al., 2001). More recently, the focus
has also been on composting of plant residues to pro-
duce growing media (Jensen et al., 2001; Prasad and
Maher, 2001; Garcia-Gomez et al., 2002). Major topicsin composting research have been process control and
characterisation of maturity or stability criteria. Control
of a composting process and the properties of the end
product can be achieved by at least two different strate-
gies. One strategy is to adjust process parameters, such
as moisture level, temperature or oxygen content
0960-8524/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2004.10.007
* Corresponding author. Fax: +45 6390 4394.
E-mail address: [email protected] (D.B. Dresbøll).
(Beck-Friis et al., 2001; Smars et al., 2002). Another is
to alter the starting conditions by changing the compo-
sition or type of material used so that C/N ratio or fibrecomposition is changed (Eklind and Kirchmann,
2000a,b; Eiland et al., 2001). A third strategy, which
to our knowledge has not yet been subject to experi-
ments, is to influence the composting process by altering
the time of addition of parts of the material to be com-
posted; normally all the material to be composted is in-
cluded right from the start.
Nitrogen has often been recognised as a limiting fac-tor for microbial growth and activity during the decom-
position of plant residues (Recous et al., 1995),
especially in materials with a high C/N ratio such as
wheat straw. However, experiments on the effect of
additional N supply on the decomposition of plant
residues showed different results, ranging from positive
to negative effects on the decomposition rate (Fog,
1988). Resource quality, microclimatic conditions anddecomposer efficiency are major factors regulating
1094 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101
composition and activity of decomposer communities
and hence the process of decomposition and nutrient re-
lease (Neely et al., 1991; Agren et al., 2001). Thus, the
effect of added N on decomposition may depend on the
plant material as degradation is influenced by nutrient
content and anatomical structure of the material. Param-eters such as N source and the time scale of the decompo-
sition process also influence the effect of added N.
Recous et al. (1995) found that the ratio of N immo-
bilised to C mineralised decreased with time, and sug-
gested that there was a high N demand during the first
stages of decomposition when soluble and easily degrad-
able C compounds were mineralised, while the N de-
mand was lower when the more recalcitrant Ccompounds were decomposed.
Since much C from plant residues such as straw mate-
rials is only slowly available to microorganisms, leading
to low growth efficiency, a limited amount of N may be
required during decomposition, and recycling of N may
then be adequate to meet the N requirements (Bremer
et al., 1991). Microorganisms, especially fungi, have a
considerable capacity to adapt to N deficient conditions.A large amount of N initially could consequently result
in immobilisation. This greater N immobilisation may
depend on (1) synthesis of microbial biomass with a lower
C/N ratio; (2) higher N losses; or (3) reduced N minerali-
sation or re-mineralisation, which may have been related
to reduced microbial activity (Bremer et al., 1991).
When composting material with the purpose of creat-
ing a growing medium it is important to understand themineralisation and immobilisation processes, as nutrient
release is controlling plant growth. Availability of nutri-
ents from organic composts is often limited despite a
high initial nutrient input, and considerable nutrient
losses frequently occur during the composting period,
primarily due to gaseous emissions. As nutrients are a
limited resource in organic production a more efficient
nutrient use is desirable. Many horticultural plants arevery nutrient demanding and compost used as fertiliser
should provide a high nutrient level from the start. It
was hypothesised that such high efficiency composts
Table 1
The setup for experiments I and II including amounts of material added to e
ratios and water contents of the mixtures
Wheat straw (kg) Clover-g
Experiment I
Treatment 1: all material present initially 32 48
Treatment 2: 1/3 clover-grass initially, 2/3
after 3 weeks
32 16
Treatment 3: 1/6 clover-grass initially, 5/6
after 3 weeks
32 8
Experiment II
Treatment 1: all material present initially 35.5 21.5
Treatment 2: 1/4 clover-grass initially,
3/4 after 3 weeks
35.5 5.5
could be prepared by splitting the addition of the nutri-
ent rich material during the composting process. The
first addition at the start of the composting process
should be sufficient to support the turnover of the read-
ily available carbohydrates. The remaining nutrient rich
material should be added later in the process when theturnover of the wheat straw would already be proceed-
ing. Decomposition of the newly added material would
then result in less N immobilisation compared to com-
post produced by a single addition at the beginning of
the process.
The objective of this study was to test this hypothesis,
by comparing turnover and N release in composts pre-
pared in the above mentioned way with composts pre-pared with all the material present initially.
2. Methods
2.1. Experimental design and materials
Compost was made of wheat straw as structural com-ponent and clover-grass hay as a nutrient rich compo-
nent. The wheat straw was air dried after harvest,
whereas the clover-grass hay was shredded into pieces
of <20mm and oven dried after harvest. Total C and
N of the materials were determined, the wheat straw
having a C/N ratio of approximately 100kg/kg and the
clover-grass hay a C/N ratio of 15kg/kg. Initial C/N
ratio of the composts was calculated based on theamount of organic material and the total C and N con-
tent of the materials.
Two experiments were set up as summarised in Table
1. Experiment I had three different treatments with three
replicates in each. Treatment 1 was a mixture of clover-
grass hay and wheat straw giving a C/N ratio of 25.
Treatment 2 had only one third of the clover-grass hay
from the beginning and thus an initial C/N ratio of 38.Treatment 3 had even less, only one sixth of the clover-
grass hay initially, which resulted in a C/N ratio of 50.
After three weeks of composting, when temperature
ach compost box in the different treatments, and the initial C/N mass
rass initial (kg) Clover-grass 3 weeks (kg) C/N Water (%)
0 25 78
32 38 79
40 50 77
0 35 54
16 60 52
D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101 1095
had decreased to 20 �C indicating less microbial activity
in treatments 2 and 3, supplemental clover-grass hay
was added in these treatments. Hence, in the end all
three treatments had received the same amount of clo-
ver-grass hay and wheat straw in total.
In experiment II only two different treatments withthree replicates were compared as no significant differ-
ences were found between treatments 2 and 3 at the
end of experiment I. The N content was reduced in this
experiment as compost from experiment I was too nutri-
ent rich as a growing medium. Treatment 1 was a mix-
ture of wheat straw and clover-grass hay giving a C/N
ratio of 35. Treatment 2 contained only a quarter of
the clover-grass hay initially resulting in a C/N ratio of60. Three weeks later the remaining clover-grass hay
was added.
The wheat straw was cut into pieces of <10cm and
the substrates were mixed in a compost mixer and
watered to a water content of approximately 70%.
Leaching losses during mixing were avoided by placing
some of the straw material beneath the mixer to absorb
runoff. After mixing, the straw material was added tothe compost boxes. Additional water was added when-
ever necessary during the composting time to keep the
water contents above 50%. The composting experiments
were performed in 800L wooden boxes measuring
0.7 · 1.0 · 1.2m (height · width · length). Heat loss
was minimised by insulation with glass–wool mats and
the boxes were passively aerated by heat convection.
Total composting time was 7 12weeks in experiment I
and 8 weeks in experiment II. After three weeks, the
compost of both experiments was turned and watered
in the compost mixer and the supplemental clover-grass
was added.
2.2. Sampling
Every 1 12-weeks, samples were taken for analysis of
water content, mineralised N, total N and C content,
and pH in experiment I. In experiment II sampling
was conducted every week. Five samples of approxi-
mately 100g were collected from each box in experiment
I, pooled and mixed. Subsamples were taken from the
pooled sample for each of the analyses. In order to
improve sampling techniques ten smaller samples of
approximately 5g were taken from each compost boxat a depth of 0.2–0.5m and pooled for each analysis in
experiment II. The entire sample was used for the ana-
lysis. In this way, each analysis was conducted on mate-
rial randomly collected throughout the compost matrix
and should provide an average of the compost in the
box. Samples were collected randomly from the boxes
except for the peripheral zone of 0.1m in both experi-
ments. At the beginning, after three weeks and at termi-nation of the experiments all boxes were weighed to
determine the total weight loss. Temperature was mea-
sured continuously in the centre of the composting
boxes using standard acid proof stainless steel Pt-100
probes connected to a data logger (Datataker DT500).
2.3. Physical and chemical analysis
Water content was determined by weight loss of com-
post samples, which were oven dried at 80 �C for 24h.
Total N and C were measured by dry combustion of
dried and finely ground samples with an automated
N–C analyser interfaced with an isotope mass spectro-
meter (Carlo Erba, EA 1108). Samples were acidified
by pouring 60ml 0.1M HCl over the entire sample be-
fore drying to avoid NHþ4 losses.
The actual concentrations of NHþ4 and NO�
3 in the
compost were measured after each sampling. Compost
samples (20g fresh weight) were analysed for NHþ4
and NO�3 content in a 2M KCl extract (compost: solu-
tion ratio 1:10) followed by shaking for 45min and cen-
trifugation. The supernatant was filtered through
Advantec 6 (Frisenette Aps.) filters and stored frozen.
As the extracts had a dark colour due to organic acids,they were cleared by shaking the extract with active C
for 15min followed by filtration (0.45lm pore size) to
avoid interference in spectrophotometric measurements.
The analysis for NHþ4 and NO�
3 content was conducted
by standard colorimetric methods using flow-injection
analysis (FIA) (Keeney and Nelson, 1982). Ammonium
was measured by high pressure liquid chromatography
(HPLC) in experiment I. pH was measured in a solutionof compost (20g fresh weight) and water in a ratio of
1:5.
2.4. Statistics
The results were calculated as an average of three rep-
licates of each treatment. Data were log transformed to
obtain homogeneity of variance and analysed with theGLM procedure of the SAS statistical package (SAS
Institute Inc., Cary, NC, USA).
3. Results
3.1. Process parameters
3.1.1. Temperature
In treatment 1 of experiment I temperature increased
rapidly to thermophilic conditions peaking at 68 �C (Fig.
1a). Temperatures remained above 40 �C for more than
three weeks. Turning the compost did not result in a
temperature increase in this treatment. In treatment 1
of experiment II the initial increase in temperature to
thermophilic conditions only lasted less than a week(Fig. 1b). In treatments 2 and 3 of experiment I the ther-
mophilic phase lasted 8–10 days, after which the temper-
Time (weeks)
0 2 4 6 8 10
Tem
pera
ture
(oC
)
0
20
40
60
80
123
Time (weeks)
0 2 4 6 8 10
12
(a) (b)
Fig. 1. Temperature development during composting in (a) experiment I: treatment 1: all material present initially (——), treatment 2: 1/3 clover-
grass hay added initially, 2/3 after 3 weeks (� � �), and treatment 3: 1/6 clover-grass hay added initially, 5/6 after 3 weeks (- - -). (b) shows temperature
development in experiment II: treatment 1: all material present initially (——), and treatment 2: 1/4 clover-grass hay added initially, 3/4 after 3 weeks
(� � �).
1096 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101
ature continued to decrease. After addition of the sup-plementary clover-grass hay the temperature increased
again to 70 �C followed yet again by a fast decline
(Fig. 1a). The temperature pattern in treatment 2 of
experiment II resembled the pattern of treatments 2
and 3 of experiment I although the temperature maxi-
mum was significantly lower in treatment 2 of experi-
ment II.
3.1.2. Compost pH
pH varied between 7.6 and 8.9 in treatment 1 of
experiment I and the highest values were found after
3–4 weeks (Fig. 2a), when the NHþ4 concentration was
also high. Towards the end of the experiment the pH de-
clined to 8.4. Treatments 2 and 3 followed the same pat-
tern, with the exception that after 3 weeks when the
additional clover-grass hay was added the pH declined
Time (weeks)
0 2 4 6 8
pH
6
7
8
9
10123
(a)
Fig. 2. Changes in pH during composting of wheat straw and clover-grass
treatment 2: 1/3 clover-grass hay added initially, 2/3 after 3 weeks (s), and tre
(b) experiment II: treatment 1: all material present initially (d), and treatmen
are the mean of three replicates and bars show the standard deviation.
to 6.6–6.7. In treatment 1 of experiment II pH varied be-tween 7.3 and 8.9, whereas pH in treatment 2 varied be-
tween 7.5 and 8.5. Towards the end of the composting
period a small decrease was observed in treatment 1
(Fig. 2b).
3.1.3. Weight loss
After three weeks of composting, before additional
clover-grass was added, no significant differences werefound in weight losses between the treatments in exper-
iment I (44–45% of initial weight). In experiment II
weight losses after three weeks were only half the losses
in experiment I and as in experiment I no significant dif-
ference between the treatments was observed (Table 2).
After 7 12weeks weight losses in all treatments were
about 61–63% of initial weight in experiment I, whereas
the weight losses in experiment II after 8 weeks were
Time (weeks)
0 2 4 6 8
12
(b)
hay in (a) experiment I: treatment 1: all material present initially (d),
atment 3: 1/6 clover-grass hay added initially, 5/6 after 3 weeks (.) and
t 2: 1/4 clover-grass hay added initially, 3/4 after 3 weeks (s). Results
Table 2
Weight and carbon losses after three weeks of composting and at the end of the composting experiments, and N losses at the end of the experiments
3 Weeks Final
Weight loss (%) C loss (%) Weight loss (%) C loss (%) N loss (%)
Experiment I
Treatment 1: all material present initially 46 (6) 50 (7) 61 (6) 60 (6) 4 (16)
Treatment 2: 1/3 clover-grass initially, 2/3 after 3 weeks 45 (5) 52 (6) 63 (5) 63 (6) 12 (10)
Treatment 3: 1/6 clover-grass initially, 5/6 after 3 weeks 44 (7) 52 (8) 61 (1) 62 (2) 9 (3)
Experiment II
Treatment 1: all material present initially 16 (5) 24 (11) 48 (9) 54 (3) 30 (24)
Treatment 2: 1/4 clover-grass initially, 3/4 after 3 weeks 26 (8) 32 (3) 54 (5) 58 (5) 22 (11)
Results are means of three replicates. Standard deviations are shown in parentheses.
Time (weeks)0 2 4
C/N
rat
io
0
20
40
60
12
C/N
rat
io
0
20
40
60
80
123
(a)
(b)
6 8
Fig. 3. Changes in C/N ratio during composting of wheat straw and
clover-grass hay in (a) experiment I: treatment 1: all material present
initially (d), treatment 2: 1/3 clover-grass hay added initially, 2/3 after
3 weeks (s), and treatment 3: 1/6 clover-grass hay added initially, 5/6
after 3 weeks (.) and (b) experiment II: treatment 1: all material
present initially (d), and treatment 2: 1/4 clover-grass hay added
initially, 3/4 after 3 weeks (s). Results are the mean of three replicates
and bars show the standard deviation.
D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101 1097
48% in treatment 1 and 54% in treatment 2; however, the
difference was not statistically significant (Table 2).
3.2. Carbon and nitrogen loss
Total loss of C was determined from the changes in
compost total C content over the experimental periods.
After three weeks of composting no significant differ-ences were observed between the treatments in either
of the experiments. After 7 12weeks the C losses did
not vary significantly between the three treatments in
experiment I whereas the difference between the two
treatments in experiment II after 8 weeks was more pro-
nounced although not statistically different (Table 2). As
observed in the weight losses, the C losses were signifi-
cantly higher in experiment I compared to experimentII.
Nitrogen losses were determined as the mass balance
of N and reflected both N gaseous emissions and N
leaching. As the water content was regulated carefully
throughout the composting period the losses due to
leaching were minimal. Nitrogen losses in experiment I
were low with less than 13% of total N lost during the
composting period. In experiment II higher N losseswere observed in both treatments with up to 30% losses
(Table 2). No significant differences were observed
between the treatments in either of the experiments.
The percentage of C in the compost of both experiments
was quite constant at around 46% during the compo-
sting period, although there was a slight tendency to a
reduction in the C concentration (results not shown).
The percentage of N increased from 2.8% to 4.6% intreatment 1 of experiment I whereas it increased from
1.7–1.8% to 4.6% in the other treatments. In experiment
II the increase was less steep from 1.3% to 2.2% in treat-
ment 1 and from 0.8% to 2.5% in treatment 2. These re-
sults are reflected in the C/N ratios (Fig. 3).
3.3. Nitrogen mineralisation
Postponing the addition of some of the nutrient rich
material had a significant effect on the timing and extent
of the mineralisation processes in experiment I. When all
the clover-grass hay was added initially, the NHþ4 con-
centration increased during the first weeks, and declined
afterwards (Fig. 4b). The NO�3 concentration remained
low for the first three weeks, followed by a steady
Tot
al in
orga
nic
N (
µg g
-1 D
W)
0
1000
2000
3000
4000
NH
4+-N
(µg
g-1
DW
)
0
1000
2000
3000
4000
123
Time (weeks)1 2 3 4 5 6 7 8
NO
3--N
(µg
g-1
DW
)
0
1000
2000
3000
4000
(a)
(b)
(c)
Fig. 4. Mineralisation pattern during composting of wheat straw and
clover-grass hay in experiment I: (a) total mineralised nitrogen, (b)
ammonium content, and (c) nitrate content. Treatment 1: all material
present initially (d), treatment 2: 1/3 clover-grass hay added initially,
2/3 after 3 weeks (s), and treatment 3: 1/6 clover-grass hay added
initially, 5/6 after 3 weeks (.). Results are means of three replicates
and bars show the standard deviation.
Tota
l ino
rgan
ic N
(µg
g-1 DW
)
0
50
100
150
200
Time (weeks)
0 2 4 6 8 10
NO
3- -N (µ
g g-1
DW
)
0
50
100
150
200
NH
4+ -N
(µg
g-1 D
W)
0
50
100
150
200
12
(a)
(b)
(c)
Fig. 5. Mineralisation pattern during composting of wheat straw and
clover-grass hay in experiment II: (a) total mineralised nitrogen, (b)
ammonium content, and (c) nitrate content. Treatment 1: all material
present initially (d), and treatment 2: 1/4 clover-grass hay added
initially, 3/4 after 3 weeks (s). Results are means of three replicates
and bars show the standard deviation.
1098 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101
increase (Fig. 4c). Postponing the addition of some of
the clover-grass hay resulted in an altered mineralisation
pattern. The NHþ4 concentration increased slightly dur-
ing the first three weeks followed by an increase when
the rest of the clover-grass hay was added (Fig. 4b).
After five to six weeks a decline in NHþ4 was observed.
At the same time a steep increase in NO�3 concentration
was seen after six weeks of no NO�3 production (Fig. 4c).
At the end of the experiment the total mineralised N
concentration was significantly higher (p < 0.05) after
the postponed addition (Fig. 4a). After 7 12weeks the
inorganic nitrogen content in the compost in treatments
2 and 3 (Fig. 4) corresponded to about 3.5% of the ini-
tial total N being mineralised, whereas only about 1.6%
of initial total N was mineralised in treatment 1. In
experiment II only a small amount of NHþ4 was detected
during the composting period, whereas practically no
NO�3 was found (Fig. 5b and c), hence after 8 weeks
the inorganic content corresponded to 0.15% of the ini-
tial amount of total N being mineralised. After 7 12weeks
of composting in experiment I, the NHþ4 =NO�
3 ratios
were 0.24, 0.32 and 0.25 for treatments 1, 2 and 3 respec-
D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101 1099
tively. In contrast, after 8 weeks of composting the
NHþ4 =NO�
3 ratios in experiment II were much higher,
2.4 in treatment 1 and 3.6 in treatment 2.
3.4. Sampling
Variation between replicates was high, and altering
the sampling procedure did not minimise the variation.
This could probably be explained by uncertain determi-
nation of the dry matter content of the total amount of
compost in the boxes, influencing most of the measured
parameters.
4. Discussion
The mineralisation in treatment 1 in experiment I fol-
lowed a pattern often observed during decomposition in
compost (Eklind and Kirchmann, 2000b). During the
microbially very active initial phase NHþ4 can be accu-
mulated, which can result in an elevation of the pH, as
mineralisation of organic N is a proton assimilating pro-cess (Beck-Friis et al., 2003). This was observed in both
experiments. The combination of high pH, high NHþ4
concentrations and high temperatures promote NH3
volatilisation and the highest ammonia losses occur dur-
ing this phase (Witter and Lopez-Real, 1987; Martins
and Dewes, 1992; Beck-Friis et al., 2003). The following
decrease in pH coincided with NO�3 production as the
nitrification process releases protons.Nitrate contents were not measurable until after the
initial three weeks of composting as high temperatures
inhibit the nitrifying bacteria (Willers et al., 1998).
Nitrifying bacteria probably survived in the peripheral
zone of the composting boxes where the temperature
was not as high as in the centre of the compost pile.
Nitrate content in the compost increased after the tem-
perature decreased and the compost was turned, sug-gesting that nitrifying bacteria were mixed into the
compost.
Despite the presence of conditions permitting ammo-
nia losses the total N losses were low, 4–29% of initial N
content. Compared to what is normally seen during
composting these losses were relatively small. During
composting of animal manure, household waste, and
other waste products, losses of at least 50% may beobserved (Witter and Lopez-Real, 1988; Martins and
Dewes, 1992; Eklind and Kirchmann, 2000b). The low
N losses in these experiments could partly be explained
by the experimental set-up as the compost was not
rotated and aerated as much as compost in reactors
(Eklind and Kirchmann, 2000a; Beck-Friis et al., 2003)
or in heaps (Martins and Dewes, 1992; Sommer,
2001). More importantly, the materials used had low ini-tial NHþ
4 contents compared to many waste materials
with high NHþ4 contents or easily degradable com-
pounds such as urea and thus a higher risk of losses
when temperature and pH increase (Noble et al., 2002).
Postponing the addition of some of the nutrient rich
material altered the mineralisation patterns significantly
in experiment I. During the first three weeks the NHþ4
content was low as no net mineralisation occurred,probably because inorganic N was immobilised during
the degradation of soluble and easily degradable carbo-
hydrates. Recous et al. (1995) observed a decrease in
the ratio of N immobilised to C mineralised with time,
confirming the high initial N demand. When the sup-
plemental clover-grass hay was added after three weeks
an increase in NHþ4 content was observed, indicating
that the organic N from the clover-grass hay amend-ment was mineralised and N immobilisation was lower
than in treatment 1. These results support the hypo-
thesis that a limited amount of N is needed initially
in the decomposition of the readily available carbohy-
drates of the straw material (Bremer et al., 1991). Usu-
ally, bacteria degrade the soluble compounds during
the initial phases of decomposition, whereas fungi with
a higher C/N ratio decompose more recalcitrant com-pounds (Recous et al., 1995; Klamer and Baath,
1998). The fungal/bacteria index increases during the
decomposition of material with a high initial C/N ratio.
The same phenomenon occurs during the initial phases
of composting (Eiland et al., 2001), confirming the fun-
gal dominance in degrading recalcitrant compounds.
When the additional N was added the readily available
carbohydrates were presumably already degraded andless N demanding fungi dominated the decomposition.
Thus, when the N was mineralised from the supplemen-
tal clover-grass hay it was not re-immobilised by the
microbial population to the same degree as when all
clover-grass hay was added initially. Therefore the de-
layed addition of clover-grass hay resulted in a higher
total release of inorganic N during the experimental
period. The NO�3 content, however, remained low for
three weeks more than in treatment 1.
The initial N level was reduced in experiment II,
resulting in a N level which was probably so low that al-
most no net mineralisation occurred. Only a small net
production of NHþ4 occurred during the first three weeks
in treatment 1, after which immobilisation was detected
(Fig. 5a and b). During the first three weeks, treatment 1
of experiment II was comparable to treatment 2 ofexperiment I, having similar initial C/N ratios. Hence,
presumably sufficient N was available for the initial bac-
terial decomposition of soluble compounds in treatment
1 of experiment II. The increase in temperature as well
as the C and weight losses indicated considerable micro-
bial activity. During decomposition of plant material
in soil, Recous et al. (1995) observed that if mineral N
was not available the C decomposition decreased butit did not stop. Despite the low N content in treatment
1 of experiment II the C mineralisation proceeded
1100 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101
throughout the experimental period, suggesting that the
initial low N content could have altered the microbial
succession. Decomposition might have been dominated
by fungi which may have the ability of effective remobil-
isation and transfer of N to actively growing parts
(Cowling and Merrill, 1966). As no supplemental clo-ver-grass hay was added in treatment 1 of experiment
II no considerable net N mineralisation was observed
after the initial three weeks.
Total C losses resembled losses normally seen during
decomposition of plant material (Bremer et al., 1991). It
could have been expected that the mass and C losses
would be lower in treatments 2 and 3 than in treatment
1 in experiment I, with the delayed addition of nutrientrich material, since the decomposition was expected to
proceed more slowly but steadily after the initial decom-
position of the readily available carbohydrates. How-
ever, no decrease of the mass and C loss was seen in
treatments 2 and 3 compared to treatment 1. This could
be explained by the higher microbial activity in treat-
ments 2 and 3 when the supplementary clover-grass
hay was added, indicated by the temperature increase.In addition, a small increase in temperature normally
occurs when the compost is turned due to better aera-
tion and humidity conditions or simply due to the whirl.
During composting the C/N ratio decreased to
around 10 in all three treatments of experiment I, which
indicated the biological stability of the composts (Bernal
et al., 1996). The C/N ratio can be used as a compost
maturity parameter implying a stable organic mattercontent and absence of phytotoxic compounds (Bernal
et al., 1998). Another maturity parameter is the ratio
between NHþ4 -N and NO�
3 -N as decreasing amounts
of NHþ4 -N combined with increases in NO�
3 -N concen-
trations towards the end of composting suggest that
intensive biological decomposition has been completed
(Pare et al., 1998). This shift in inorganic N was seen
in experiment I, although the ratio was just above thesuggested maturing index of 0.16 (Bernal et al., 1996).
In experiment II on the other hand, the C/N ratio never
declined to less than 20 and no NO�3 was produced, con-
firming that decomposition became N limited. Addition-
ally, the high NHþ4 =NO�
3 ratio is indicative of immature
compost.
5. Conclusions
In conclusion, postponing the addition of nutrient
rich material affected the mineralisation pattern result-
ing in more plant available N after 7 12weeks of compo-
sting. Total losses of mass, C and N were not affected
significantly by the delayed addition. This suggests that
without altering the amount or type of material to becomposted, mineralisation can be managed by simple
methods, which could be of great importance when
growing plants with a high initial N demand. There
seemed, however, to be a critical N addition below
which net mineralisation was not obtained.
Acknowledgement
We thank Stig Sandholt Andersen, Jens Barfod and
Birthe Flyger for skilful technical assistance as well as
Hanne Lakkenborg Kristensen and Jakob Magid for
valuable comments on the manuscript. Financial sup-
port was provided by the Danish Research Centre for
Organic Farming (DARCOF).
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